1. Field of the Invention
The present invention relates to an ink-jet recording apparatus for performing a recording operation by ejecting an ink from a recording head to a recording medium, and a temperature control method of the ink-jet recording apparatus.
2. Related Background Art
Recording apparatuses such as printers, copying machines, facsimile apparatuses, and the like record an image consisting of a dot pattern on a recording medium such as a paper sheet or a plastic thin plate on the basis of image information.
The recording apparatuses can be classified into ink-jet type, wire-dot type, thermal type, laser beam type, and the like according to their recording systems. Of these apparatus, an ink-jet type apparatus (ink-jet recording apparatus) causes a recording head to eject a flying ink (recording liquid) droplet from an ejection port thereof, and attaches the ink droplet to a recording medium to perform a recording operation.
In recent years, a large number of recording apparatuses are used, and are demanded to satisfy requirements such as high-speed recording, a high resolution, high image quality, low noise, and the like. As a recording apparatus, which can satisfy these requirements, the above-mentioned ink-jet recording apparatus is known. In the ink-jet recording apparatus, since a recording operation is performed by ejecting an ink from a recording head, stabilization control of an ink ejection operation, and an ink ejection quantity, which is necessary for satisfying the above-mentioned requirements, is largely influenced by the temperature of the recording head.
For this reason, the conventional ink-jet recording apparatus adopts so-called closed-loop control, i.e., a method wherein an expensive temperature sensor is provided to a recording head unit, and based on the detected temperature of the recording head, the temperature of the recording head is controlled within a desired range or ejection restoration processing is controlled. As a heater for the temperature control, a heater member joined to the recording head unit, or an ejection heater is used in an ink-jet recording apparatus, which forms a flying droplet by utilizing a heat energy to perform recording, i.e., in an apparatus for ejecting an ink droplet by the growth of a bubble caused by film boiling of an ink. When the ejection heater is used, it must be energized to a temperature as low as a bubble non-forming temperature.
In particular, in a recording apparatus for obtaining an ejection ink droplet by forming a bubble in a solid or liquid ink using a heat energy, closed-loop temperature control is generally performed since ejection characteristics considerably change depending on the temperature of the recording head, as is conventionally known. Otherwise, a low-cost type printer, which completely ignores printing quality, density nonuniformity, and the like, and is used in a compact electronic calculator, can only be available.
However, with the advent of portable OA apparatuses represented by lap-top personal computers, a portable printer is also required to have high quality. As for portable printers, due to their compact design structures, an exchangeable cartridge type head, in which a head and an ink tank are integrated, is expected to become increasingly popular. In addition, the exchangeable cartridge type head is also expected to become popular from the viewpoint of maintenance due to the popularity of home/personal use wordprocessors, personal computers, and facsimile apparatuses.
In this case, however, since a temperature sensor, a heater, and the like for temperature control are incorporated in the exchangeable cartridge, the following drawbacks are posed.
(1) Variation in temperature control measurement value due to variation in temperature sensor
Since exchangeable heads are expendable supplies, every time a head is exchanged, a sensor suffering from a variation in characteristics is connected when viewed from the printer main body side.
In a recording head for forming a flying droplet by utilizing a heat energy to perform recording, since an ejection heater is manufactured in a semiconductor process, it is indispensable to build a diode sensor for detecting the temperature of the recording head in the same process from the viewpoint of a decrease in cost. Since the diode sensor suffers from a variation in the manufacture, it does not have precision as high as a temperature sensor as a selected product. Thus, the surrounding temperatures measured by diode sensors in different manufacturing lots sometimes have a difference of 15.degree. C. or more.
For this reason, in closed-loop temperature control using the temperature sensor of the recording head, a variation in temperature sensor of the recording head must be adjusted in an extra adjustment step, or after a temperature sensor, which is ranked by measurement, is attached to the main body, it is corrected by an adjustment switch, thus requiring troublesome adjustment operations.
These adjustment operations considerably increase manufacturing cost, and deteriorate operability. Also, an increase in signal processing amount due to these adjustment operations, and a large increase in processing amount of an MPU due to the closed-loop control itself impose heavy loads on the apparatus design of compact, portable type printer main bodies.
(2) Countermeasure against electrostatic noise
Since exchangeable heads are expendable supplies, a user repetitively attaches/detaches the head from the main body. For this reason, contacts of the main body apparatus side are always exposed.
Since the output from a temperature sensor is directly supplied from the exchangeable head to a circuit on a printed circuit board of the main body through a carriage and flexible wiring lines, a temperature measurement circuit is very weak against electrostatic noise. This weak point is enhanced since the housing of a compact, portable printer cannot have a sufficient shield effect.
Therefore, in a conventional temperature detection method, electrostatic shields and parts as a countermeasure against electrostatic noise must be added for only one temperature sensor, and a compact structure, a decrease in cost, and quality are considerably damaged.
(3) Time delay
The object of temperature detection of the recording head is to control the temperature of the recording head within a desired range, and to perform stabilization control of the recording ink ejection operation, and the ejection quantity, as described above. More specifically, temperature detection of the recording head means detection of the ink temperature on the ejection heater in a strict sense. However, since it is difficult to directly detect the ink temperature on the ejection heater, the temperature sensor is attached near the heater (or nozzle) (the mounting position of the temperature sensor will be described in detail later). In an ink-jet recording apparatus, since the heat conduction speed of a heater board is lower than the speed of a change in ink temperature near the ejection heater, a time delay from an actual temperature is generated even if the temperature of the head is continuously detected.
Since the above-mentioned control is to feed back a temperature detected by the temperature sensor to a heating amount by the heater, the time delay disturbs precise control.
(4) Temperature detection error
In temperature detection by the temperature sensor, a temperature may be erroneously detected due to a thermal flow or electrical noise input to the temperature sensor. In order to prevent this, a method of averaging several detection values of the head temperature, and determining an average value as a current head temperature is adopted. However, when several detection temperatures are averaged, the following problems are posed:
[1] dynamic changes in temperature of the recording head are averaged; and PA1 [2] a time delay is generated between an actual temperature and a detection value. Thus, these problems disturb precise feedback control.